What Are Resonant Converters?

Resonant converters are a class of DC-DC power conversion circuits that incorporate a resonant tank, typically a network of one or more inductors and capacitors, to shape the current and voltage waveforms at the switching transitions. By allowing the switches to commutate when the tank voltage or current passes through zero (zero voltage switching, or zero current switching), resonant converters reduce the energy dissipated in semiconductor devices at each switching event, enabling operation at higher frequencies and achieving higher efficiency than conventional hard-switched pulse-width modulated converters. Resonant converters draw from circuit theory, electromagnetics, and power electronics, and are used wherever high power density, high efficiency, or low electromagnetic interference are required.

The principal advantage of resonant operation is the elimination or reduction of switching losses. In a hard-switched converter, a MOSFET or IGBT turns on or off while carrying full voltage or current, producing a transient power spike that heats the device and radiates electromagnetic interference. Resonant circuits shape the switching transients so that the device commutates near a zero crossing, reducing both the thermal and electromagnetic burden on the system.

Series and Parallel Resonant Topologies

The foundational resonant topologies are the series resonant converter (SRC) and the parallel resonant converter (PRC), defined by how the resonant tank connects relative to the load. In the SRC, the load is in series with the tank, so the output current is filtered by the resonant network and the converter behaves as a current source with load-dependent characteristics. In the PRC, the load is in parallel with the resonant capacitor, making the converter behave more like a voltage source but requiring higher circulating currents under light load conditions. Both topologies regulate output by varying the switching frequency relative to the tank's resonant frequency, creating a variable-impedance filter whose attenuation changes with frequency. The SRC loses regulation at no load because the series path blocks current; the PRC maintains no-load regulation but carries reactive current continuously. ScienceDirect's review of resonant converter classification for electric vehicle applications surveys these topologies and their trade-offs in terms of voltage gain, conduction loss, and dynamic behavior.

LLC Resonant Converters

The LLC resonant converter, which takes its name from the three reactive elements in its tank (series inductor L, parallel inductor L, and series capacitor C), has become the dominant isolated resonant topology in commercial power supply design. The converter uses a half-bridge or full-bridge switch network to drive a square wave into the LLC tank, which filters harmonics and delivers a near-sinusoidal waveform to the transformer and rectifier. Primary MOSFETs achieve zero voltage switching at turn-on under all load conditions, while secondary rectifier diodes achieve zero current switching, eliminating reverse recovery losses. LLC converters routinely achieve 93 to 96% efficiency at full load. A 2015 IEEE conference paper reviewing LLC resonant converter design covers frequency-domain analysis, voltage gain curves, and the trade-offs between magnetizing inductance, leakage inductance, and output capacitance that determine the converter's operating range. The LLC topology has largely supplanted earlier phase-shifted full-bridge designs in server power supplies and high-power adapters for its combination of efficiency and simplicity.

Control and Wide-Range Operation

Resonant converters are controlled by modulating switching frequency relative to the tank resonant frequency rather than by varying duty cycle. Below resonance, the converter exhibits inductive impedance and achieves zero voltage switching; above resonance, the impedance becomes capacitive and zero current switching becomes available, though with different loss characteristics. Frequency-modulated control is implemented with variable-frequency oscillators and can be combined with burst-mode control at light loads to maintain efficiency when the converter would otherwise carry high circulating reactive current for minimal delivered power. An IEEE Xplore analysis of zero voltage switching conditions in LLC converters addresses how secondary parasitic capacitances affect the boundary between ZVS and hard switching.

Applications

Resonant converters have applications in a wide range of fields, including:

• Server and data center power supplies, where LLC converters deliver 80 Plus Titanium efficiency
• Electric vehicle on-board chargers and DC fast-charging stations
• Consumer electronics adapters and laptop chargers
• Renewable energy power conditioning for solar inverters and battery storage systems
• Medical imaging power supplies requiring low EMI and high isolation